The present invention relates to improved polymerization catalyst compositions of the Ziegler-Natta type, procatalysts for use in forming such catalyst compositions, methods of making such catalyst compositions and procatalysts, and to methods of using the catalyst compositions to make an olefin polymer.
Ziegler-Natta olefin polymerization catalyst compositions typically comprise a solid component containing magnesium, titanium and halide moieties in combination with an internal electron donor (which combination is referred to as the xe2x80x9cprocatalystxe2x80x9d), a substance that is capable of converting the procatalyst to an active polymerization catalyst (referred to as a xe2x80x9ccocatalystxe2x80x9d), and a selectivity control agent (SCA) or external donor. Suitable internal electron donors especially include aromatic mono- or di-alkylesters or ether derivatives thereof, such as alkylbenzoates, dialkylphthalates, and C1-4 alkyl ether derivatives thereof. Conventional cocatalysts include aluminum trialkyls, such as triethylaluminum or triisobutylaluminum. The cocatalyst may be combined or complexed with some or all of the internal electron donor, selectivity control agent, or both, if desired. Although variations in any of these catalyst components will influence the performance of the resultant catalyst, the component that appears to offer the greatest opportunity for modification to produce greater catalyst activity is the procatalyst.
Various methods of preparing procatalysts are previously disclosed in the patent art. Examples include: U.S. Pat. Nos. 5,247,032, 5,247,031, 5,229,342, 5,153,158, 5,151,399, 5,146,028, 5,124,298, 5,106,806, 5,082,907, 5,077,357, 5,066,738, 5,066,737, 5,034,361, 5,028,671, 4,990,479, 4,927,797, 4,829,037, 4,816,433, 4,728,705, 4,548,915, 4,547,476, 4,540,679, 4,535,068, 4,472,521, 4,460,701, 4,442,276, and 4,330,649. One preferred method from among the foregoing disclosures is a method of forming a xe2x80x9cprocatalyst precursorxe2x80x9d from a mixture of magnesium dialkoxides and titanium alkoxides and reacting the mixture with titanium tetrachloride in the presence of an alcohol, an aromatic hydroxide compound, and an aromatic solvent, especially chlorobenzene. In this manner, a solid material is recovered by selective precipitation upon removal of alcohol from the solution. This precursor may thereafter by contacted with an internal electron donor and washed with TiCl4 in a halohydrocarbon solvent to form the desired procatalyst. Among the foregoing disclosures, U.S. Pat. No. 5,124,298 and U.S. Pat. No. 5,082,907, disclose that an acid chloride, such as benzoyl chloride or phthaloyl chloride, may be used in combination with the TiCl4 and a halohydrocarbon in at least one such wash step to further facilitate the replacement of at least a portion of the alkoxide moieties. In U.S. Pat. No. 4,535,068 benzoyl chloride was contacted with a magnesium alkoxide precursor compound both during preparation of a procatalyst and in a subsequent step. The reference stated that the benzoyl chloride contacting step may occur either before or simultaneously with halogenation by means of a tetravalent titanium halide.
According to the present invention, there is provided a method of making a solid procatalyst composition for use in a Ziegler-Natta olefin polymerization catalyst composition, said method comprising:
contacting a solid precursor composition comprising magnesium, titanium, and alkoxide moieties with a titanium halide compound and an internal electron donor in any order, in a suitable reaction medium to prepare a solid procatalyst composition,
separating the solid procatalyst composition from the reaction medium,
further exchanging residual alkoxide functionality of the solid procatalyst composition for chloride functionality by contacting the same two or more times with benzoyl chloride under metathesis conditions for a period of time sufficient to prepare a solid procatalyst composition having a decreased alkoxide content compared to the alkoxide content of the solid procatalyst composition before said exchange, and
recovering the solid procatalyst composition.
Also included in the present invention are the solid procatalysts resulting from the foregoing methods of preparation; olefin polymerization catalysts comprising one or more of the foregoing procatalyst compositions, a cocatalyst, and optionally a selectivity control agent; an improved olefin polymerization process comprising contacting an olefin monomer under olefin polymerization conditions in the presence of the foregoing catalyst composition; as well as polyolefin polymers formed thereby.
The catalyst compositions of the present invention are useful in preparing xcex1-olefin polymers having relatively high bulk density. Moreover, they enable the preparation of polypropylene impact copolymers, especially polypropylene that is impact modified by ethylene/propylene copolymers prepared in situ, having increased rubber contents, at elevated polymerization temperatures, without disadvantageous formation of polymer particle agglomerates.